Optical fiber coating systems commonly comprise two coating compositions. The first coating composition contacts the glass surface and is called the inner primary coating. The second coating composition is designed to overlay the inner primary coating and is called the outer primary coating.
The inner primary coating is usually a soft coating having a low glass transition temperature (hereinafter "Tg"), to provide resistance to microbending. Microbending can lead to attenuation of the signal transmission capability of the coated optical glass fiber and is therefore undesirable. The outer primary coating is typically a harder coating providing desired resistance to handling forces, such as those encountered when the coated fiber is cabled.
For the purpose of multi-channel transmission, optical fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical fiber assemblies include ribbon assemblies and cables. A typical optical fiber assembly is made of a plurality of coated optical fibers which are bonded together in a matrix material. For example, the matrix material can encase the optical fibers, or the matrix material can edge-bond the optical fibers together.
Optical fiber assemblies provide a modular design which simplifies the installation and maintenance of optical fibers by eliminating the need to handle individual optical fibers.
Coated optical fibers for use in optical fiber assemblies are usually coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to facilitate identification of the individual coated optical glass fibers. Such ink coatings and colored outer primary coatings are well known in the art. Thus, the matrix material which binds the coated optical fibers together contacts the outer ink layer if present, or the colored outer primary coating.
When a single optical fiber of the assembly is to be fusion connected with another optical fiber, or with a connector, an end part of the matrix layer is required to be stripped away from the optical fiber. A common method for practicing ribbon stripping at a terminus of the ribbon assembly is to use a heated stripping tool. Such a tool consists of two plates provided with heating means for heating the plates to about 90 to about 120 C. An end section of the ribbon assembly is pinched between the two heated plates and the heat of the tool softens the matrix material and the primary coatings prior to and during the stripping procedure.
Ideally, the primary coatings on the coated optical fibers, and the ink coating if present, are removed simultaneously with the matrix material to provide bare portions on the surface of the optical fibers (hereinafter referred to as "ribbon stripping"). In ribbon stripping, the matrix material, primary coatings, and ink coating, are desirably removed as a cohesive unit to provide a clean, bare optical glass fiber which is substantially free of residue. Any residue can interfere with the optical glass fiber ribbon mass fusion splicing operation, and therefore is presently removed by wiping with a solvent prior to splicing. However, the solvent wipe can cause abrasion sites on the bare optical fiber, thus compromising the integrity of the connection. Many attempts have been made to increase the strip cleanliness of the ribbon assemblies by adding adhesion reducing additives to the inner primary coating which results in systems with little improvement in the strip cleanliness or system with insufficient adhesion. The ability to produce ribbon assemblies that can be stripped to provide clean, residue-free, bare optical glass fibers without unduly sacrificing other desirable or required properties of the primary coatings continues to challenge the industry.
There are many test methods which may be used to determine the performance of a ribbon assembly during ribbon stripping. An example of a suitable test method for determining the stripping performance of a ribbon is disclosed in the article by Mills, G., "Testing of 4- and 8- fiber ribbon strippability", 472 International Wire & Cable Symposium Proceedings (1992), the complete disclosure of which is incorporated herein by reference.
Many attempts have been made to understand the problems associated with ribbon stripping and to find a solution to increase ribbon stripping performance. The following publications attempt to explain and solve the problems associated with ribbon stripping: K. W. Jackson, et. al., "The Effect of Fiber Ribbon Component Materials on Mechanical and Environmental Performance", 28 International Wire & Symposium Proceedings (1993); H. C. Chandon, et. al., "Fiber Protective Design for Evolving Telecommunication Applications", International Wire & Symposium Proceedings (1992); J. R. Toler, et. al., "Factors Affecting Mechanical Stripping of Polymer Coatings From Optical Fibers", International Wire & Cable Symposium Proceedings (1989); and W. Griffioen, "Strippability of Optical Fibers", EFOC & N, Eleventh Annual Conference, Hague (1993).
The ability of a ribbon assembly to ribbon strip cleanly so as to provide bare optical glass fibers that are substantially free of residue was heretofore unpredictable and the factors affecting ribbon stripping not fully understood. Accordingly, there is a need for an optical fiber, radiation-curable coating composition system that improves the strippability of optical fiber ribbons.